A display device using a light-emitting element in which an organic compound is used as a light-emitting substance (an organic EL element) has been developed rapidly as a next generation display device because it has advantages such as thinness, lightness in weight, high response speed, and low power consumption. Although there have been various obstacles, technique has been improved such that organic EL televisions have become commercially available recently.
In an organic EL element, when voltage is applied between a pair of electrodes which interpose a light-emitting layer therebetween, electrons and holes injected from the electrodes are recombined to form an excited state, and when the excited state returns to a ground state, light is emitted. A wavelength of light emitted from a light-emitting substance is peculiar to the light-emitting substance; thus, by using different types of organic compounds as light-emitting substances, light-emitting elements which exhibit various wavelengths, i.e., various colors can be obtained.
In the case of a display device which is expected to display images, such as a display, at least three colors of light, i.e., red, green, and blue are required to be obtained in order to reproduce full-color images. To achieve this, for example, there are following methods: a method in which a color filter is combined with a light-emitting element emitting light with a light-emitting spectrum in a wide wavelength, a method in which a color conversion layer is combined with a light-emitting element emitting light with a wavelength shorter than a wavelength of an objective color, and a method in which a light-emitting element emitting light with a desired wavelength is used. Among these three methods, the final one, i.e., a method in which an objective color is obtained directly is preferable because loss in energy is small in this method.
This method is employed for the above organic EL televisions which have become commercially available; however, actually, in addition to that method, a color filter is used, and a micro cavity structure is employed for a light-emitting element in order to improve color purity. Organic EL televisions having got many advantages are naturally expected to provide high quality images as a next generation television, and light-emitting elements exhibiting appropriate emission colors are required to meet the expectation.
A light emitted from a light-emitting substance is peculiar to the substance as described above. There are many measures to improve the color purity of an organic EL television, which means that it is very difficult to obtain a light-emitting element which exhibits light emission of a favorable color and also satisfies another important property such as a lifetime or power consumption. In addition, an important property of a light-emitting element, such as a lifetime or power consumption, does not necessarily depend only on a substance exhibiting light emission. The property is greatly affected also by layers other than a light-emitting layer, an element structure, an affinity between a light-emitting substance and a host, or the like. Therefore, it is true that many kinds of materials are necessary for light-emitting elements in order to show the growth of this field. As a result thereof, materials for light-emitting elements which have a variety of molecular structures have been disclosed (for example, see Reference 1).